A disk cassette wherein a magnetic disk, such as a flexible disk, for computers, or another disk that is operated in a magnetic manner, electrostatic manner, optical manner, or another manner, is housed in a case made of a resin and the like, is provided with an opening section through part of the case. This opening section can be opened or closed by a shutter that can be slid to the left or right so that a head, such as a magnetic head, is accessible to the disk.
Conventionally, the material used to manufacture the disk shutter is stainless steel or synthetic resin. However, the disk shutters wherein these materials are used are accompanied by the following problems.
First, a shutter made of a stainless steel is heavy and high in cost. In addition, a shutter made of stainless steel lacks satisfactory affinity with printing ink when letters (characters) or the like are printed on the surface, and there is also a problem on the adhesion of the printed letters.
On the other hand, a shutter using a synthetic resin is readily charged with static electricity, to attach dust and dirt, leading to a problem that adversely affects the disk in the case. Further, like a shutter made of a stainless steel, the printability is poor. In addition, there is also a problem on the dimensional accuracy of the shutter.
In recent years, as materials used to make the disk shutter, aluminum alloys have been proposed. JU-A-1-181171 ("JU-A" means unexamined published Japanese utility model application) and JP-A-6-111516 ("JP-A" means unexamined published Japanese patent application) propose, as a shutter material, aluminum alloy materials each provided with an oxide film (Alumite or Alumirite film) on anode, and JP-A-6-346178, JP-A-9-192601, JP-A-8-104939, and JP-A-8-161849 propose resin-coated aluminum alloy materials.
However, the aluminum alloy materials provided with an oxide film on anode have the problem that the film is apt to be cracked at the part where the aluminum alloy material is bent at an angle of 90.degree., and peeling of the film and the contact of the aluminum alloy material with a mold bring about tailings (residual substances) when the aluminum alloy material is worked with the mold. Further, it may occur that the cracks spread (are propagated) to the base material, and as a result the aluminum alloy material breaks at the part where it is bent at an angle of 90.degree..
On the other hand, with respect to the resin-coated aluminum alloy materials, JP-A-6-346178 propose forming an ethylene acrylic-series, epoxy-series, or epoxyacrylic-series resin coating film containing a given amount of an inner wax, and Japanese patent application No. 8-23142(1996) proposes forming an ionomer resin coating film containing a given amount of a petroleum wax.
However, in these conventional resin-coated aluminum alloy materials, when they are manufactured into articles desired through a variety of steps, such as a press-forming step and an assembling step, scratches and/or bruises are liable to occur during the handling by a worker or the transfer or transportation of the articles, and thus the commercial value thereof is lowered considerably, in some cases.
To improve the flaw resistance of resin-coated aluminum alloy materials, an attempt is made to secure the film hardness by adding colloidal silica to the film, as proposed in JP-A-8-104939, or an attempt is made to improve the flaw resistance by applying a silicate compound containing silica (SiO.sub.2) and M.sub.2 O, wherein M is Li, Na, or K, as proposed in JP-A-8-161849. However, even these are not satisfactory in flaw resistance in fact. These resin-coated aluminum alloy materials have such problems that the part where the material is bent at an angle of 90.degree. is apt to break, tailings easily occurs during continuous forming and/or working and the tailings readily accumulate during the operation, and the adhesion of printing ink at the time of printing is not satisfactory.